1. Field of the Invention
The present invention is related to the inspection of structures, and more particularly to nondestructive inspection of structures accomplished through analysis of sound waves from the structure.
2. Related Art
Many of our nation""s bridges and other similar structures are aging rapidly. As a consequence, it has become increasingly important to determine the structural integrity of concrete structures such as concrete bridge decks. Known systems and methods for testing concrete structures are expensive, slow or tedious.
One problem of particular concern is the detection of concrete delamination. Concrete delamination is the separation of a concrete structure into two or more layers. Most concrete structures, such as bridge decks, employ reinforcing bar (referred to herein as rebar) for structural strength. In a structure such as a bridge deck, it is customary to lay the rebar in a grid pattern approximately two inches below the surface of the concrete. The location of the rebar is also frequently the location of delamination. The delamination occurs when water comes into contact with the rebar, causing it to corrode. Because corrosion is an expansive process, the rebar acts as a wedge that splits the concrete into layers.
Some of the earliest techniques for locating delamination in concrete bridge decks involve either tapping on the surface of the bridge deck with a hammer or metal rod or dragging a chain bar across the bridge deck. A clear ringing sound is produced by an intact, healthy structure. However, where a delamination exists, a dull hollow sound is heard. These techniques have the advantage of being simple and inexpensive. They allow inspectors to inspect large structures in a shorter amount of time than is possible with some other nondestructive testing techniques, such as ultrasonic pulse-velocity methods. However, these techniques have the disadvantages of (a) relying on the suggestive interpretation of the inspector; and (b) being difficult to implement in noisy environments, such as inspecting one lane of a bridge deck while traffic is driving in the other lanes.
Some state of the art systems use ground penetrating radar to locate defects such as delamination. These systems have inherent problems, especially with ghosting and signature overlap. These systems are often prohibitively expensive as well. Other non-destructive techniques, such as ultrasonic pulse-velocity, suffer from similar drawbacks.
Another known device, which is referenced in ASTM Standard Practice D4580-86, xe2x80x9cStandard Practice for Measuring Delaminations in Concrete Bridge Decks by Sounding,xe2x80x9d involves impulse excitation and the use of piezo-electric hydrophones to detect defects. The hydrophones are enclosed in a soft, oil-filled tire which is mounted on a mobile cart. The hydrophones are mounted inside the tire such that they are in close proximity to the concrete but do not rotate when the tire rotates. The hydrophones are physically coupled to the concrete through the soft tires and the oil in the tires. Properly calibrating and maintaining this device has proven problematic. Another problem occurs when using this device over the grooves commonly cut into concrete bridge decks today. This problem is a result of the device""s reliance on physical coupling between the sensor and the surface.
What is needed is a simple and inexpensive method for inspecting concrete structures that does not rely on the subjective interpretation of an human inspector.
The aforementioned needs are met to a great extent by the present invention, which provides a method for inspecting concrete structures in which characteristics of a signal from a structure of known quality are compared to characteristics of an unknown signal. In one embodiment of the invention, a distance measurement is used to compare the two signals. The distance measurement is then used an indicator. The distance measurement, or indicator value, is then thresholded and the result is a yes/no decision or a variable where value is an indicator of quality. The comparison and generation of the indicator value can be performed using autonomous learning techniques such as artificial neural networks, transform spaces such as, but not limited to, Linear Prediction Coefficients or Cepstral Coefficients, through direct filtering methods, or through frequency domain comparisons. The characteristics of a known structure can be compiled from a database which averages the characteristics from among many samples (which may include both high quality and low quality structures).
A preferred embodiment of the invention includes the steps of exciting the structure to be inspected with a mechanical device such as a chain, sensing resulting vibrations with a sonic receiver, such as a microphone, not physically coupled to the structure, processing the received signals to exclude signals with frequencies outside of a range of response frequencies corresponding to defects desired to be detected, and comparing the processed signals to a threshold. Areas for which the energy across the frequency bands of interest exceeds the threshold correspond to defects such as delaminations. The received signals are collected from various locations on the structure. In preferred embodiments, the location of the received signals is also maintained. The location may be obtained through use of a device such as an odometer or a differential global position sensor (DGPS).
The invention also provides an apparatus for inspecting concrete structures. In preferred embodiments the apparatus comprises a mobile platform, such as a wheeled cart or trailer. A chain bar, which is a horizontal bar that includes several chains, preferably of equal length, attached to it is mounted to the cart such that the chains drag along the structure to be inspected when the mobile platform is moved. A microphone is attached to the mobile platform and positioned such that it preferentially receives sounds generated by the excitation source on the surface under test. The signals received by the microphone are then converted to digital form by a data conversion device and input to a processor. The processor then processes the signals to exclude signals corresponding to frequencies outside of the desired frequency range and compares the processed signals to a threshold. Also attached to the processor is a position sensing device such as an odometer or DGPS. The processed data is output to an output device, which is a chart recorder or a computer display in preferred embodiments. Both raw and processed data are also stored in preferred embodiments. The raw data may be used for archival and/or training purposes, or for post-processing.
In a highly preferred embodiment, results are displayed in a plan view format in a graph in which a horizontal axis represents position along a length of the structure, a vertical axis represents position along a width of the structure, and the energy from the frequencies of interest is indicated by color. Positions on the structure at which the energy falls below a threshold are not displayed in such embodiments.
An aspect of the invention is that it is more robust because the acoustical sensor is not physically coupled to the structure to be tested. Another aspect of the invention is the automation of the detection process through the comparison of the received energy to a threshold. Still another aspect of the invention is the ability to process the acoustical data and locate defects in real time. This aspect allows defects to be marked on the structure when the defects are detected in some embodiments.
These and other aspects, advantages and features of the present invention can best be understood with reference to the drawings and accompanying description herein.